This invention relates to new and useful improvements in containers, and more particularly to a method of forming a container having enhanced sidewall crystallinity and low base crystallinity. The container is particularly adapted for use as a refillable carbonated beverage container able to withstand higher caustic wash temperatures and exhibit reduced product flavor carryover, or as a hot fill container.
The market for PET refillable carbonated soft drink (CSD) bottles has enjoyed significant growth worldwide since its introduction in 1987 by Continental PET Technologies. These bottles have been commercialized throughout much of Europe, Central and South America, and are now moving into the Far East market.
Refillable bottles reduce the existing landfill and recycle problems associated with disposable plastic beverage bottles. In addition, a refillable bottle provides a safer, lighter-weight plastic container in those markets, currently dominated by glass, where legislation prohibits use of non-returnable packages. The goal is to produce a refillable bottle having the necessary physical characteristics to withstand numerous refill cycles, and which is still economical to produce.
Generally, a refillable plastic bottle must maintain its functional and aesthetic features over a minimum of 10 and preferably over 20 cycles or loops to be considered economically feasible. A loop is comprised of (1) an empty hot caustic wash followed by (2) contaminant inspection and product filling/capping, (3) warehouse storage, (4) distribution to wholesale and retail locations and (5) purchase, use and empty storage by the consumer followed by eventual return to the bottler. This cycle is illustrated in FIG. 1. In an alternative cycle, the contaminant inspection occurs prior to the caustic wash.
Refillable containers must meet several key performance criteria to achieve commercial viability, including:
1. high clarity (transparency) to permit on-line visual inspection;
2. dimensional stability over the life of the container; and
3. resistance to caustic wash induced stress cracking and leakage.
A commercially successful PET refillable CSD container is presently being distributed by The Coca-Cola Company in Europe (hereinafter xe2x80x9cthe prior art containerxe2x80x9d). This container is formed of a single layer of a polyethylene terephthalate (PET) copolymer having 3-5% comonomer, such as 1,4-cyclohexanedimethanol (CHDM) or isophthalic acid (IPA). The preform, from which this bottle is stretch blow molded, has a sidewall thickness on the order of 5-7 mm, or about 2-2.5 times that of a preform for a disposable one-way bottle. This provides a greater average bottle sidewall thickness (i.e., 0.5-0.7 mm) required for abuse resistance and dimensional stability, based on a planar stretch ratio of about 10:1. The average crystallinity in the panel (cylindrical sidewall section beneath the label) is about 15-20%. The high copolymer content prevents visual crystallization, i.e., haze, from forming in the preform during injection molding. Preform haze is undesirable because it produces bottle haze which hinders the visual on-line inspection required of commercial refill containers. Various aspects of this prior art container are described in Continental PET Technology""s U.S. Pat. Nos. 4,725,464, 4,755,404, 5,066,528 and 5,198,248.
The prior art container has a demonstrated field viability in excess of 20 refill trips at caustic wash temperatures of up to 60xc2x0 C. Although successful, there exists a commercial need for an improved container that permits an increase in wash temperature of greater than 60xc2x0 C., along with a reduction in product flavor carryover. The latter occurs when flavor ingredients from a first product (e.g., root beer) migrate into the bottle sidewall and subsequently permeate into a second product (e.g., club soda) on a later fill cycle, thus influencing the taste of the second product. An increase in wash temperature may also be desirable in order to increase the effectiveness and/or reduce the time of the caustic wash, and may be required with certain food products such as juice or milk.
Thus, it would be desirable to increase the caustic wash temperature above 60xc2x0 C. for a returnable bottle having a lifetime of at least 10 refill trips, and preferably 20 refill trips, and to reduce the product flavor carryover. These and other objects are achieved by the present invention as set forth below.
In accordance with this invention, a method of forming a container is provided having an enhanced level of sidewall crystallinity and a low level of base crystallinity. The container has improved resistance to caustic stress cracking, while maintaining a high level of transparency (clarity) and dimensional stability, and thus is particularly suitable for refillable beverage bottles. The container has a lifetime of at least 10 refill cycles and more preferably at least 20 refill cycles, at caustic washing temperatures of above 60xc2x0 C. The container exhibits a reduction in flavor carryover of at least 20% over the previously described refillable CSD prior art container.
The method of forming the container includes a first expansion step in which a substantially amorphous polyester preform is at least partially expanded into an intermediate article, followed by a heat treating step in which the intermediate article is at least partially heated to contract and crystallize the same, and then a second expansion step in which the contracted intermediate article is reexpanded to form the final container.
In a first method embodiment of the invention, a base-forming section of the preform is not expanded during the first expansion step, is not heated and remains substantially unchanged in crystallinity during the heat treating step, and is expanded without significant crystallinity change during the second expansion step. In contrast, a sidewall-forming section of the preform is expanded during the first expansion step to dimensions substantially equal to or greater than the dimensions of the final container sidewall, heated to crystallize and contract the same below the dimensions of the final container during the heat treating step, and reexpanded during the second expansion step to the final dimensions of the container sidewall. The relatively thinner container sidewall thus achieves a substantially higher percent crystallinity than the thicker base, which provides enhanced resistance to caustic stress cracking in both the sidewall and base.
In a second method embodiment, the base-forming section of the preform is expanded during the first expansion step, but is not heated during the heat treating step so that it maintains a low level of crystallinity compared to the container sidewall. Again, the sidewall-forming section of the preform is expanded during the first expansion step to form an intermediate expanded sidewall with dimensions substantially equal to or greater than the dimensions of the final container sidewall, the expanded intermediate sidewall is then heated to crystallize and contract the same below the dimensions of the final container sidewall, and then the contracted intermediate sidewall is expanded during the second expansion step to the final dimensions of the container sidewall. The thinner container sidewall thus achieves a substantially higher percent crystallinity than the thicker base, which provides enhanced resistance to caustic stress cracking in both the sidewall and base.
The base-forming section of the preform is generally substantially thicker than the sidewall-forming section and thus more resistant to heating (and resultant crystallization) during the heat treating step. In addition, it is preferred to localize or confine the heat treatment to the intermediate sidewall, while the base-forming section (or base) is shielded to prevent heating thereof. In one preferred heat treating step, the intermediate container is heated by passing through a row of heating elements and shielding elements move (or increase in size) to protect the base-forming section (or base) as it moves upwardly with the contracting sidewall. In addition, a contracting centering rod is positioned within the contracting intermediate article, and the internal pressure within the intermediate article is controlled, to promote uniform and controlled contraction thereof. In another preferred heat treating step, a cooling mechanism such as a movable water-cooled base cup remains in contact with the base-forming section (or base) to prevent heating thereof. Alternatively, a cooling mechanism directs a cooling fluid (such as cold air) against the base-forming section (or base) of the contracting article to prevent heating of the base. In addition, the relatively thicker neck and shoulder sections may be shielded to prevent heating thereof.
The resulting container has a highly oriented, relatively thin and highly crystalline sidewall panel portion having at least 25% average crystallinity, and more preferably about 30 to 35% average crystallinity. The container base includes a thickened base portion of low orientation and crystallinity, i.e., no greater than about 10% average crystallinity. The wall thickness of the thickened base portion is generally at least 3xc3x97, and more typically about 3 to 4xc3x97 that of the panel. Higher crystallinity levels in the panel allow higher wash temperatures, e.g., 65xc2x0 or 70xc2x0 C., but require longer processing times (to heat and cool the sidewall). A very high crystallinity level of 50% has been achieved. By xe2x80x9caveragexe2x80x9d crystallinity is meant an average taken over the entire area of the respective container part, i.e., panel or thickened base portion.
In one embodiment, the container is a one-piece refillable pressurized beverage container with a free-standing base. The sidewall (in particular the panel) has a wall thickness of about 0.5 to about 0.8 mm, and during the first expanding step the sidewall-forming section of the preform is stretched at a planar stretch ratio of about 10-16:1 (i.e., the thickness reduction ratio of the expanded intermediate sidewall to the preform sidewall), and during the second expansion step the contracted intermediate sidewall is stretched at a planar stretch ratio of about 7-15:1, and more preferably 9-11:1 (i.e., the thickness reduction ratio of the final sidewall to the preform sidewall). The container has a champagne base with an upwardly radially increasing arcuate outer base wall, a lowermost chime, and a recessed central dome, the chime preferably having an average percent crystallinity of no greater than about 10%, and more preferably about 2-8%, and the central dome preferably having an average crystallinity of no more than about 8%, and more preferably no more than about 2%.
Alternatively, the container may have a substantially thinner xe2x80x9cfootedxe2x80x9d base including a hemispherical bottom wall with downwardly extending legs which terminate in lowermost supporting feet. The hemispherical bottom wall includes radial ribs between the legs. A relatively thin outer portion of the base (including the ribs, legs and feet) preferably has an average crystallinity of at least about 10%, and more preferably about 15-20%, and a substantially thicker central portion of the bottom wall (without legs) has an average crystallinity of no more than about 8%, and preferably no more than about 2%.
In still another embodiment, the improved resistance to stress cracking and dimensional changes at elevated temperatures makes the container of this invention particularly suitable as a hot-fill container.
These and other features of the invention will be more particularly described by the following detailed description and drawings of certain specific embodiments.